1. Field of the Invention
This invention pertains to the field of viscous petroleum recovery.
2. Description of the Prior Art
This invention is an improved method for the recovery of oil from subterranean hydrocarbon bearing formations wherein the oil is very viscous, that is, it has a low API gravity or is a bitumen. This method is especially useful for recovering hydrocarbons from reservoirs such as tar sand formations.
The recovery of very viscous oil from formations and bitumens from tar sands has generally been difficult if not impossible on a commercial scale. Although some advances have been realized in recent years in stimulating the recovery of heavy oils, i.e., oils having an API gravity in the range of 10.degree. to 25.degree. API, little success has been realized in recovering bitumens from tar sands. Bitumens are generally regarded as being highly viscous oils having a gravity in the range of about 4.degree. to 10.degree. API and are contained in an essentially unconsolidated sand referred to as a tar sand. Vast quantities of tar sand exists in the Athabasca region of Alberta, Canada. Although these deposits contain several hundred billion barrels of oil or bitumen, the recovery of this bitumen using conventional in situ techniques has been less than successful. The reasons for this lack of success relates primarily to the fact that bitumen is extremely viscous at the temperature of the formation with consequently low mobility. In fact, the bitumen is so viscous that it appears to be a soft solid. In addition, these tar sand formations have very low permeability even though they are unconsolidated.
Using the principal that the viscosity of oil decreases with an increase in temperature, prior art techniques have usually been designed with the idea of raising the temperature of the bitumen in situ. This improves its mobility and therefore its amenability to recovery. These thermal recovery techniques generally include steam injection and hot water injection as well as in situ combustion.
Usually these techniques employ an injection well and a production well spaced apart from each other and penetrating an oil bearing formation. In the usual steam operation involving two wells, the steam is introduced into the formation through the injection well and the heat from the steam is transferred to the bitumen (if a tar sand is involved) thus lowering its viscosity and therefore improving mobility while the flow of the hot fluid in the injection well drives the bitumen toward the production well from which it may be produced.
Normally, in an in situ combustion operation, an oxygen containing gas, such as air is introduced into the formation through an injection well and combustion of the in place crude adjacent to the well bore is initiated by one of many known means such as the use of a downhole gas fired heater or a downhole electric heater or in some cases chemical means. Thereafter, the injection of oxygen containing gas is continued to maintain a combustion front which is formed, and to drive the front through the formation toward the production well.
Ideally, as the combustion front advances through the formation, a swept area is formed consisting of a clean sand matrix behind the front. Ahead of the advancing front various contiguous zones are formed and are also displaced ahead of the combustion front. These zones may be envisioned as a distillation and cracking zone near the front, a vaporization and condensation zone farther from the front, an oil bank even farther from the front, and lastly an unaltered zone.
The temperature at the combustion front is generally very high ranging from 650.degree. to 1200.degree. F. The heat thus generated in this zone is transferred to the distillation and cracking zone just ahead of the combustion front where the crude or bitumen undergoes some distillation and cracking. In this zone a sharp thermal gradient is thought to exist wherein the temperature drops from the temperature of the combustion front to about 300.degree. to 450.degree. F. As the front progresses through the formation, the temperature of the formation continues to rise and the heavier molecular weight hydrocarbons of the oil become carbonized and are deposited on the matrix of the formation. These carbonized hydrocarbons are the potential fuels to sustain the progressive in situ combustion zone.
Ahead of the distillation and cracking zone is a vaporization and condensation zone. This zone is a thermal plateau and its temperature is in the range of from about 200.degree. to about 450.degree. F depending upon the distillation characteristics of the fluid in the formation and the formation pressure. These fluids consist of water and steam and hydrocarbon components of the crude or bitumen.
Ahead of the vaporization and condensation zone is an oil bank which fills up as the in situ combustion front progresses and the formation of crude is displaced toward the production well. This zone is highly oil saturated but contains not only reservoir fluids but also condensate, cracked hydrocarbons and gases which are products of combustion which eventually reach the production well from which they may be produced.
Although in situ combustion has been used to increase recovery of bitumen and viscous crudes, variations of the technique have taken place in order to improve its performance, for example, water or saturated steam is sometimes injected with the air. See for example, U.S. Pat. No. 2,584,606. This is sometimes referred to as wet combustion. This has improved the process somewhat. However, the method has several weaknesses which will limit the process to only a very few reservoirs. It has been found, for example, that the wet process is restricted to relatively heavy crudes containing very high molecular weight hydrocarbons, thick reservoirs and very close well spacing, which contribute to very high costs.
In addition, U.S. Pat. No. 2,839,141 suggests that super heated steam injection and in situ combustion with super heated steam is a way to displace heavy oils. However, this method also has limitations. Even though it conducts a great deal of heat initially into the formation, it cannot displace all of the oil in the swept zone and since the super heated zone cannot propagate over great distances from the well bore, it also requires close well spacing.
Laboratory models utilizing simultaneous injection of super heated steam and air have recovered over half of the bitumen in place. Although these results are an improvement over the simple wet in situ combustion, it has the same limitations as the separate method, that is, it leaves behind in the swept zone a significant quantity of combustible material. There is always a significant degree of vertical permeability variation especially in tar sand reservoirs, which causes the thermal front to migrate through only a portion of the oil saturated interval. As a result heat loss is high which prevents the thermal front from propagating at great distances from the injection well. In the case of in situ combustion, the combustion front will finally cease when the vertical combustion interval narrows down to about 4 feet.
Our invention proposes a method which will be an improvement over prior art methods in that it will eliminate many of the disadvantages which render them ineffective in some cases. The objectives of our invention are to increase the distances of the propagation of very high temperature fronts thereby reducing the necessity for a large number of wells, to increase the efficiency of the thermal method and to increase the thermal conformance in both the vertical and horizontal planes.